Average high latitude magnetic field: Variation with interplanetary sector and with season—I. Disturbed conditions

High latitude magnetic field data from 16 northern observatories are averaged during periods of magnetic disturbance level Kp = 2^? to 3⁺. Within this disturbance level, variations between interplanetary magnetic field sector (toward and away from the Sun) and geomagnetic season (dipole latitude of the Sun: > 10° = summer, < ? 10° = winter) are delineated. Variations between seasons are: (1) The positive bay and polar cap disturbance is a maximum in summer and a minimum in winter for both sectors. (2) The negative bay disturbance is a maximum in summer and a minimum in winter when the interplanetary field is toward the Sun and vice versa during away sectors. Variations between sectors are: (1) During summer and equinox the negative bay disturbance is greater for toward sectors than for away sectors. The reverse occurs during winter. (2) The positive bay disturbance is greater during toward sectors than during away sectors for all seasons. (3) All diiferences in disturbance level are greater at sunlit local times than in darkness. (4) Angular differences in the direction of the horizontal disturbance of up to 75° occur between sectors in the polar cap and dayside during all seasons. (5) The polar cap-auroral belt boundary location is different for the two sectors. Compared to data from away sectors, this boundary for toward sectors is shifted northward near dawn (5–8^h) and southward between 10 and 22^h. (6) Accompanying this boundary difference there is a change in the direction of the vertical disturbance in the region between 9 and 14^h at geomagnetic latitudes 77–88°. ΔZ in this region is negative during away sectors and positive during toward sectors. Differences between sectors are attributed to changes in the ionospheric electric field configuration and in the distribution of magnetic field aligned currents.Features unrelated to sector or season also occur: (1) A significant Y component is present in both the positive and negative bays. (2) The vertical disturbance $\text{(¦ΔZ¦)}$ to the north of the auroral belt is much larger than that to the south. (3) Two distinct regions of maximum activity are present in the ΔZ accompanying the positive bay disturbance.     

Sharp Changes of Solar Wind Ion Flux and Density Within and Outside Current Sheets

Analysis of the Interball-1 spacecraft data (1995 – 2000) has shown that the solar wind ion flux sometimes increases or decreases abruptly by more than 20% over a time period of several seconds or minutes. Typically, the amplitude of such sharp changes in the solar wind ion flux (SCIFs) is larger than 0.5×10⁸ cm⁻² s⁻¹. These sudden changes of the ion flux were also observed by the Solar Wind Experiment (SWE), on board the Wind spacecraft, as the solar wind density increases and decreases with negligible changes in the solar wind velocity. SCIFs occur irregularly at 1 AU, when plasma flows with specific properties come to the Earth’s orbit. SCIFs are usually observed in slow, turbulent solar wind with increased density and interplanetary magnetic field strength. The number of times SCIFs occur during a day is simulated using the solar wind density, magnetic field, and their standard deviations as input parameters for a period of five years. A correlation coefficient of ∼0.7 is obtained between the modelled and the experimental data. It is found that SCIFs are not associated with coronal mass ejections (CMEs), corotating interaction regions (CIRs), or interplanetary shocks; however, 85% of the sector boundaries are surrounded by SCIFs. The properties of the solar wind plasma for days with five or more SCIF observations are the same as those of the solar wind plasma at the sector boundaries. One possible explanation for the occurrence of SCIFs (near sector boundaries) is magnetic reconnection at the heliospheric current sheet or local current sheets. Other probable causes of SCIFs (inside sectors) are turbulent processes in the slow solar wind and at the crossings of flux tubes.     

The dipole-quadrupole cycle of the background solar magnetic field

The evolution of the background magnetic field with the solar cycle has been studied using the dipole-quadrupole magnetic energy behaviour in a cycle. The combined energy of the axisymmetric dipole, non-axisymmetric quadrupole, and equatorial dipole is relatively lowly variable over the solar cycle. The dipole field changed sign when the quadrupole field was near a maximum, andvice versa. A conceptual picture involving four meridional magnetic polarity sectors proposed to explain these features may be in agreement with equatorial coronal hole observations. The rate of sector rotation is estimated to be 8 heliographic degrees per year faster than the Carrington rotation (P = 27.23^d synodic). Polarity boundaries of sectors located 180° apart show meridional migrations in one direction, while the boundaries of the other two sectors move in the opposite direction. A simple model of how the magnetic field energy varies, subject to specifying reasonable initial photospheric magnetic and velocity field patterns, follows the observed evolution of the dipole and quadrupole field energies quite nicely.     

Thermal proton flow in the plasmasphere: The morning sector

Vertical profiles of electron density obtained in the vicinity of the plasmapause using the Alouette-II topside sounder have been analyzed to assess the presence of H⁺ flow in the topside ionosphere. The observations in the midnight sector show clearly the presence of the plasmapause; i.e. there is a sharp boundary separating the poleward regions of polar wind H⁺ flow and the more gentle conditions of the plasmasphere where light ions are present in abundance. In contrast, in the sunlit morning sector upwards H⁺ flow is deduced to be present to invariant latitudes as low as 48° (L = 2·2) in the regions normally known to be well inside the plasmasphere. The upwards H⁺ flux is sufficiently large (3 × 10⁸ ions cm^?2 sec^?1) that the plasmapause cannot be seen in the latitudinal electron density contours of the topside ionosphere. The cause for this flow remains unknown but it may be a result of a diurnal refilling process.     

Precipitation of > 115 kev protons in the evening and forenoon sectors in relation to the magnetic activity

Data from a low altitude polar orbiting satellite, on auroral protons >115 keV in the evening and forenoon sectors, are presented.In the forenoon sector there is a weak but fairly steady precipitation at Λ ≈ 75° during quiet conditions. This precipitation is situated at higher invariant latitudes at local noon than at local dawn and can probably be ascribed to the high energy tail of the polar cleft protons. During moderately disturbed conditions, especially during the recovery phase of geomagnetic storms, there are some seemingly more “impulsive” precipitation events at Λ ≈ 65°. During very disturbed conditions these two precipitation zones in the forenoon sector seem to merge.In the evening sector a rather sharp equatorward boundary of the main precipitation, at Λ ≈ 69° during quiet conditions, varies fairly smoothly from pass to pass. South of this boundary, at invariant latitudes around 62°, there is a steady weak drizzle from the radiation belt. Due to a longitudinal effect this drizzle, as recorded by the satellite, shows a diurnal variation.The equatorward boundaries of the main precipitation at both local times move equatorward with increasing ring current strength. When D_st gets less than about — 100nT, the poleward boundaries are found to move equatorward too. From an attempt to reveal some of the substorm-dependent changes of the precipitation it is found that an equatorward shift of the precipitation areas takes place during, or just prior to, the substorm expansive phase, accompanied by a large intensity increase in the evening sector, whereas the recovery phase is linked with a poleward expansion of the precipitation at both local times.     

Low-energy particle events associated with sector boundaries

Onsets of some 40 to 45 low-energy proton events during the years 1957–1969 coincided in time with transits of well-defined sector boundaries across the Earth. These events can be interpreted as long-lived proton streams filling up some of the magnetic sectors, indicating an acceleration of protons which is not associated with typical proton-producing flares. The sharp onsets of these particle streams, as well as a deficiency of flare-associated particle events shortly before the boundary transit, indicate that in some cases magnetic sector boundaries can inhibit transverse propagation of low-energy particles in the solar corona or in interplanetary space.     

Green corona emission,calcium plage activity and solar sector magnetism

The relationship between coronal green line emission and solar sector magnetism has been studied statistically for the years 1965–1969. This period includes the rising portion and the maximum phase of solar cycle no. 20. In the years around solar maximum the results suggest the existence of longitudinal magnetic arcades at the solar sector boundaries. The arcades extend from at least 50°N to 50°S and are flanked by north-south oriented coronal holes about 90° apart. In the rising portion of the cycle the general picture consists of a high green line intensity structure to the west of the boundary and a region of low intensity several days wide to the east of it.Analyses of the calcium plage distribution in the years 1962–1969 show that, on the average, there is a tendency for the plage activity to peak near the sector boundaries. It is further concluded that the activity distribution suggested by Wilcox (1971a, b) is not typical of the behaviour of solar activity relative to the sector boundaries.     

A model combining the polar and the sector structured solar magnetic fields

A phenomenological model of the interplay between the polar magnetic fields of the Sun and the solar sector structure is discussed. Current sheets separate regions of opposite polarity and mark the sector boundaries in the corona. The sheets are visible as helmet streamers. The solar sector boundary is tilted with respect to central meridian, and boundaries with opposite polarity change are oppositely tilted. The tilt of a given type of boundary [(+, ?) or (?, +)] changes systematically during the sunspot cycle as the polarity of the polar fields reverses. Similar reversals of the position of the streamers at the limbs takes place. If we consider (a) a sunspot cycle where the northern polar field is inward (?) during the early part of the cycle and (b) a (+, ?) sector boundary at central meridian then the model predicts the following pattern; a streamer at high northern latitudes should be observed over the west limb together with a corresponding southern streamer over the east limb. The current sheet runs now NW-SE. At sunspot maximum the boundary is more in the N-S direction; later when the polar fields have completed their reversal the boundary runs NE-SW and the northern streamer should be observed over the east limb and the southern streamer over the west limb. Observational evidence in support of the model is presented, especially the findings of Hansen, Sawyer and Hansen and Koomen and Howard that the K-corona is highly structured and related to the solar sector structure.     

Response of the polar cap boundary and the current system to changes in IMF observed from the Magsat satellite in the southern hemisphere during summer

The magnetic field vector residuals observed from the Magsat satellite have been used to obtain the dependence of the polar cap boundary and the current system on IMF for quiet and mildly disturbed conditions (K_p ⩽ 3 +). The study has been carried out for the summer months in the Southern Hemisphere. “Shear reversals” (SRs) in vector residuals indicative of the infinite current sheet approximation of the field-aligned currents (FACs) indicate roughly the polar cap boundary or the poleward boundary of the plasma sheet. This is also the poleward edge of the region 1 FACs. The SR is defined to occur at the latitude where the vector goes to minimum and changes direction by approximately 180°.It is found that SRs mainly occur when the interplanetary magnetic field (IMF) has a southward-directed B_z- component and in the latitude range of about 70°–80°. SRs in the dusk sector occur predominantly when the azimuthal component B_y is positive and in the dawn sector when B_y is negative, irrespective of the sign of B_z These results agree with the known merging process of IMF with magnetopause field lines. When SRs occur on both dawn and dusk sectors, the residuals over the entire polar cap are nearly uniform in direction and magnitude, indicating negligible polar currents. Similar behaviour is observed during highly disturbed conditions usually associated with large negative values of B_z.Forty-one Magsat orbits with such SRs are quantitatively modelled for preliminary case studies of the resulting current distribution. It is found that SRs, in the plane perpendicular to the geomagnetic field, for the current vectors and the magnetic vector residuals (perturbations relative to the unperturbed field) occur at almost the same latitudes. The electrojet intensities range from 1.2 × 10⁴ to 6.5 × 10⁵ A (amperes). A preliminary classification of polar cap boundary crossings characterized by vector rotations rather than SRs also shows that they tend to occur mainly for negative B_z.     

Geomagnetic effects of interplanetary sector structure

In this paper the geomagnetic effects of the interplanetary magnetic sector structure are studied on the basis of some new criteria and working hypotheses.Thus, we assume that the recurrence of geomagnetic disturbances should be understood in a dynamical sense, in connection with the evolution of the full sector structure, and not necessarily as a 27-day recurrence. Accordingly, on the representation of the sector structure during 1968, as deduced by Wilcox and Colburn, we have defined four ‘main recurring lines’, which link the sector boundaries recurrent in successive solar rotations. The term ‘group of SC and SI events’, abbreviated as gr(SC + SI), introduced by us in previous works to designate the morphological grouping of the individual SC and SI events in collective events, is also used.It should be pointed out that the bulk of gr(SC + SI) events are either associated with sector boundaries, or recurrent in successive solar rotations. Part of these events reveal the existence of some ‘secondary recurring lines’, within the magnetic sectors.The above working hypotheses and observations have been checked by the superposed epoch analysis, performed for each main recurring line in part and for all the main recurring lines combined.The following parameters are analysed: the number of SC events, the number of collective events gr(SC + SI), the total number of SC and SI events and the geomagnetic activity index Kp.The main result of the superposed epoch analysis consists in the appearance of a sharp maximum for all the parameters considered on the day of sector boundary. This fact proves that the effects of the sector boundaries are important and general, in regard to all aspects of geomagnetic activity. Essentially these effects consist of the occurrence of gr(SC + SI) events and of a specific increase in the Kp index, when the sector boundaries pass by the magnetosphere. This suggests that the sector boundaries are accompanied by corotating shocks and magnetohydrodynamical turbulence.The high frequency in the occurrence of the SC events on the days of sector boundaries is also noticeable.Each main recurring line presents a certain ‘individuality’, expressed particularly by secondary specific maxima in all the parameters, corresponding to the ‘secondary recurring lines’. One suggests that these secondary recurring lines might be due to some corotating distortions within the magnetic sectors and might be related to the ‘subsector’ or ‘filaments’.The distribution of the geomagnetic disturbances near the sector boundaries depends on the direction of the field polarity change.All these results lead to the conclusion that most of the geomagnetic disturbances can be accounted for by the interaction between corotating distortions in the solar wind connected with the sector structure and the magnetosphere, the flare-induced disturbances representing statistically the secondary mechanism.     

The height stratification of solar magnetic fields in cycles 21–23

The radial component of the solar magnetic field, Br, was calculated in the potential approximation in the height range from 1 to 2.5 solar radii, Ro. According to these data, synoptic maps of the magnetic field for solar cycles 21–23 were constructed. For each 10-degree latitudinal zone, the proportion of its area, S _+field, that was occupied by the “+” field in each rotation was found. In the entire latitudinal zone, the radial component of the field is assumed to be positive if S_+field ≥ 80% and negative if S _+field ≤ 20%. The field proved to be virtually unipolar at the level of the photosphere (R = Ro) during most of the cycle, from the poles to the north and south latitude ≈60°. In the vicinity of minimums between cycles 21 and 22, as well as cycles 22 and 23, for a few rotations of the Sun, the field was almost unipolar within the range of latitudes (?40°)-90°. At R = 2.5 Ro, for most of each cycle, the field was unipolar in the range of latitudes (?20°-(-90°)) and (20°–90°). According to our interpretation, the shift of the polar-field boundary to the equator with height reflects superradial expansion of open magnetic flux tubes from the polar coronal holes. It was found that the reversal of the polar fields began with 1–2 rotations and ended from 2 to 14 solar rotations earlier at great heights than at the surface of the Sun. This indicates that the reversal of the large-scale field occurs first and then that of the small-scale one. In the study of the sectoral structure of the magnetic field at different heights it was found that the boundaries that rotate with a period of less than the Carrington revolution extend to greater heights than the boundaries with a Carrington or longer period. We assume that the boundaries of the first type are formed by the large-scale structures of the magnetic field and the boundaries of the second type are determined by the active regions.     

Convection and field-aligned currents, related to polar cap arcs, during strongly northward IMF (11 January 1983)

Electric and magnetic fields and auroral emissions have been measured by the Intercosmos-Bulgaria-1300 satellite on 10–11 January 1983. The measured distributions of the plasma drift velocity show that viscous convection is diminished in the evening sector under IMF B_y < 0 and in the morning sector if IMF B_y > 0. A number of sun-aligned polar cap arcs were observed at the beginning of the period of strongly northward IMF and after a few hours a θ-aurora appeared. The intensity of ionized oxygen emission [O⁺(²P), 7320 Å] increased significantly reaching up to several kilo-Rayleighs in the polar cap arc. A complicated pattern of convection and field-aligned currents existed in the nightside polar cap which differed from the four-cell model of convection and NBZ field-aligned current system. This pattern was observed during 12 h and could be interpreted as six large scale field-aligned current sheets and three convective vortices inside the polar cap. Sun-aligned polar cap arcs may be located in regions both of sunward and anti-sunward convection. Structures of smaller spatial scale correspond to the boundaries of hot plasma regions related to polar cap arcs. Obviously these structures are due to S-shaped distributions of electric potential. Parallel electric fields in these S-structures provide electron acceleration up to 1 keV at the boundaries of polar cap arcs. The pairs of field-aligned currents correspond to those S-structures: a downward current at the external side of the boundary and an upward current at the internal side of it.     

Solar radio emission at 9.1 cm and sector boundaries

A possible connection between solar radio emission from 1.0 to 9.4 GHz and the interplanetary sector boundaries has been previously reported in the literature. The present research does not support the previous work as expected. The 9.1 cm activity appears to be organized around sector boundaries only in a very limited sense in that the distribution of very strong active regions peaks near the –/+ boundaries. However, this phenomenon is only observed during the most active part of the solar cycle. A peculiar asymmetry is found regarding the length of the positive and negative sectors.     

Active solar radio regions at metric frequencies and the interplanetary sector structures

The possible relation between type I noise active regions and the polarity distribution of the interplanetary magnetic field is examined for the period from 13 March to 21 August, 1968 (Solar Rotation Numbers 1842–1847) by using data from ground-based and satellite observations. In general four type I radio regions appeared during each solar rotation period except for Rotation No. 1842. The number of type I regions is the same as the number of sector boundaries. This result suggests that the configuration of the photospheric magnetic field extending into the interplanetary space may be related to the origin of the type I radio regions. Statistically the passage of the sector boundaries is delayed by approximately 5 days after the central meridian passage of the type I noise regions on the solar disk.The position of the source of the sector boundaries and its relation to the type I radio regions are investigated by taking into account the mean bulk velocity of solar winds as observed by space probes. A model of the large-scale structure of type I radio regions and their relation to the sector structure of the magnetic field as observed in the interplanetary space is briefly discussed.NASA Research Associate at the University of Maryland.     

Evidence for a poleward meridional flow on the sun

We define for observational study two subsets of all polar zone filaments, which we call polemost filaments and polar filament bands. The behavior of the mean latitude of both the polemost filaments and the polar filament bands is examined and compared with the evolution of the polar magnetic field over an activity cycle as recently distilled by Howard and LaBonte (1981) from the past 13 years of Mt. Wilson full-disk magnetograms. The magnetic data reveal that the polar magnetic fields are built up and maintained by the episodic arrival of discrete f-polarity regions that originate in active region latitudes and subsequently drift to the poles. After leaving the active-region latitudes, these unipolar f-polarity regions do not spread equatorward even though there is less net flux equatorward; this indicates that the f-polarity regions are carried poleward by a meridional flow, rather than by diffusion. The polar zone filaments are an independent tracer which confirms both the episodic polar field formation and the meridional flow. We find:

1. The mean latitude of the polemost filaments tracks the boundary of the polar field cap and undergoes an equatorward dip during each arrival of additional polar field.
2. Polar filament bands track the boundary latitudes of the unipolar regions, drifting poleward with the regions at about 10 m s^-1.
3. The Mt. Wilson magnetic data, combined with a simple model calculation, show that the filament drift expected from diffusion alone would be slower than observed, and in some cases would be equatorward rather than poleward.
4. The observation that filaments drift poleward along with the magnetic regions shows that fields of both polarities are carried by the meridional flow, as would be expected, rather than only the f-polarity flux which dominates the strength. This leads to the prediction that in the mid-latitudes during intervals between the passage of f-polarity regions, both polarities are present in nearly equal amounts. This prediction is confirmed by the magnetic data.

     

The relationship between the slowly varying component of solar radio emission and large scale photospheric magnetic field patterns

Daily solar radio flux observations have been examined for a relationship to the large-scale photospheric magnetic field structure. Interplanetary magnetic field sector boundaries were used to indicate boundaries between photospheric field regions of opposite polarity. An enhancement in emission was found about four days before the boundary central meridian passage. Most of the effect came from emission near toward-to-away type boundaries. A higher level of emission appears to be associated with toward field regions than with away field regions.     

Simulations of the sun's polar magnetic fields during sunspot cycle 21

Regarding new bipolar magnetic regions as sources of flux, we have simulated the evolution of the radial component of the solar photospheric magnetic field during 1976–1984 and derived the corresponding evolution of the line-of-sight polar fields as seen from Earth. The observed timing and strength of the polar-field reversal during cycle 21 can be accounted for by supergranular diffusion alone, for a diffusion coefficient of 800 km² s^-1. For an assumed 300 km² s^-1 rate of diffusion, on the other hand, a poleward meridional flow with a moderately broad profile and a peak speed of 10 m s^-1 reached at about 5° latitude is required to obtain agreement between the simulated and observed fields. Such a flow accelerates the transport of following-polarity flux to the polar caps, but also inhibits the diffusion of leading-polarity flux across the equator. For flows faster than about 10 m s^-1 the latter effect dominates, and the simulated polar fields reverse increasingly later and more weakly than the observed fields.Laboratory for Computational Physics and Fluid Dynamics.E. O. Hulburt Center for Space Research.     

On the possibility of inferring the solar and interplanetary sector structure from statistics of geomagnetic storms and solar activity

Attention is drawn to the great statistical material on geomagnetic storms and solar activity, published mainly before the space age. By analyses of this material in connection with established correlations between geomagnetic activity and the interplanetary sector struc- ture, valuable information might be obtained that would significantly contribute to an increased understanding of solar and interplanetary sector magnetism.As an illustration of this, different analyses of solar-geomagnetic correlations have been considered in relation to the paper by Wilcox and Colburn (1972) on the observed sector struc- ture. Indications are found that (a) the interplanetary and solar sector pattern in the years 1919–1969 consisted of mainly 2 or 4 sectors per solar rotation, and (b) sector boundaries are related to bipolar magnetic regions on the Sun.     

Large-Scale Structures in Comet Hale–Bopp

The plasma tails of comets clearly show the demarcation of the solar wind into distinct equatorial and polar regions (Brandt and Snow (2000), Icarus 148, 52–64).The boundary is determined by the maximum extent in latitude of the heliospheric current sheet (HCS). The observational record contains many well-observed equatorial comets, but observations of comets in the polar region are relatively rare. In addition to its size and brightness, comet Hale–Bopp had an orbital inclination of 89.4° and was well observed for months in the polar region. We document the comet's large-scale appearance throughout the apparition, including the polar region and its transition into the equatorial region. The bright dust tail hampered observations of the plasma tail, particularly near the head, but images taken with a CO⁺ filter show a very large disconnection event (DE) on May 7 and May 8, 1997. The time of disconnection is estimated at approximately May 4.0. This DE is associated with a crossing of the HCS. The model calculations of the HCS indicate that other crossings might have occurred in late April, but given the uncertainty in the calculation, the comet might have missed the HCS. Sparse observational coverage and the bright dust tail prevent further investigation of the potential earlier HCS crossings. The plasma tail shows anomalous orientations at the highest latitudes and possible explanations are discussed.     

Electric Currents at imf Sector Boundaries

Numerical simulation of magnetic field reconnection at IMF sector boundaries shows that the reconnection line may be carried by the solar wind out of the region of the anomalous resistivity. This makes it possible to observe magnetic loops at the Earth's orbit open to the Sun as well as from it. Besides, it is shown that the current sheet in the vicinity of the reconnection line has to split into two currents.Experimental data on the structure of the sector boundaries are analyzed, and it is shown that the currents at sector boundaries are indeed often splitted.The thickness of the splitted boundaries may amount to 18×106 km; taking into account this value, the heliocentric distance of the region of anomalous resistivity in the interplanetary current sheet is estimated as 0.4–0.5 AU.The probability of observing magnetic loops open towards the Sun seems to be greater than that of loops open from the Sun, which suggests an essential asymmetry of the field reversal regions.